Daniel asked for more detail about how I use Google Bookmarks in Firefox.
Google Bookmarks, how do I love thee, let me count the ways...
The good old-fashioned way of course is with a bookmarklet
Bookmark pages more easily. Drag this bookmarklet to the Links area of your browser: Google Bookmark
(taken from the bottom of http://www.google.com/bookmarks - after one is logged in, of course).
Slightly more modern, there are a couple of Firefox plugins that do more than just add bookmarks. A once-and-(hopefully)-future shag of mine were lying in bed once, and in between cuddling and.. well, anyway, we looked on the net and found two plugins.
The first is the Firefox Google Bookmarks plugin. Works, kinda useful, but I found it a bit clunky. Lets you import Firefox bookmarks into Google Bookmarks; gives you a menu containing your google bookmarks...
The one I used to use (until, ooh, two days ago) was the Google Bookmarks Button. Does much the same thing, but I found the user experience to be a bit nicer.
Unfortunately I don't remember why I ended up favoring the latter; I seem to recall that it was some very small difference in UI that I preferred.
There's also GMarks, which I've not tried.
Most modern of all though - Google released version 3 of their toolbar for Firefox late last year. Taken from the Feature List:
Want to create and label bookmarks that you can access from any computer? Simply click the Toolbar’s star icon, or right-click the star to add and label a bookmark. You'll be able to access your Bookmarks menu on any computer with the new Google Toolbar installed.
Doesn't import Firefox bookmarks, but at this point I'm not using them any more. Aside from that, does much the same as the others: there's a star button to click on to add a bookmark, and a drop-down menu to show the existing ones. The Google Bookmarks Button is better in some ways - it pops up a dialog so you can edit the title of the bookmark and the tags, whereas Google's button just adds the bookmark straight in, no editing, no tags. For the moment, all I want is a way to flag pages as things that I'm going to want later, so Google's button is fine.
There's a lot that del.icio.us does that Google Bookmarks don't do. I never used any of those features - it was just a dumping ground for stuff I want want to look back on later. If you're using del.icio.us for much else, you will probably find that Google Bookmarks is far too simplistic for you.